As illustrated in FIG. 1, a mechanical arm of the prior art has a seat 11, which is mounted on a robot stand and is provided with an air or oil pressure cylinder 16. The free end of a piston rod 17 of the cylinder 16 is provided with a connection seat 14 on which a working hand 15 is mounted. The seat 11 is further provided therein with a sleeve 12 through which a guide rod 13 is pivotally put through in such a manner that the guide rod 13 is fastened at one end thereof with the connection seat 14. The connection seat 14 is thus prevented from turning aside and is capable of being push or pulled by the piston rod 13 to move linearly so as to enable the working hand 15 to be positioned for working on the workpiece. The seat 11 is further provided respectively on both sides thereof with a buffering device 18 for absorbing the impact force exerting on a position restricting block 19 located on the connection seat 14 and the guide rod 13.
Such a prior art mechanical arm as described above is defective in design in that the driving efficiency of the cylinder 16 is undermined by a relatively large area of mechanical friction between the guide rod 13 and the sleeve 12, and that the guide rod 13 or the sleeve 12 is vulnerable to deflection capable of hampering the overall performance of the mechanical arm, and further that the guide rod 13 and the sleeve 12 must be made with precision to enable the mechanical arm to perform as expected.